The catalysts normally used in polycondensation to produce polyethylene terephthalate (PET) are predominantly compounds of Antimony (Sb), mostly antimony trioxide and antimony triacetate. Germanium dioxide, though a good catalyst and gives polyester with better clarity, its usage is limited due to its high cost. Off late universally there is a tendency to shift from Sb based catalysts to non Sb catalysts primarily on the basis of its not being too eco friendly. Thus today there is always a preference to work with low levels of Sb catalyst if not totally substituting that with non Sb catalysts.
Among the various alternatives Titanium (Ti) based compounds have been widely suggested though commercially it has not replaced Sb based catalysts mainly because normally Ti imparts yellow coloration to the polyester. Variety of Ti compounds have been suggested and tried as polycondensation catalysts. These include tetra-n-propyl titanate, tetraisopropyl titanate, tetra-n-butyl titanate, tetraphenyl titanate, tetracyclohexyl titanate, tetrabenzyl titanate, tetra-n-butyl titanate tetramer, titanium acetate, titanium oxalates, sodium or potassium titanates, titanium halides, titanate hexafluorides of potassium, manganese and ammonium, titanium acetylacetate, titanium alkoxides, titanate phosphites etc.
EP 0699700, French Patent 2419949, U.S. Pat. Nos. 4,983,711, 3,962,189 and JP52062398 describe production of polyesters using Sb free Ti compounds with addition of Cobalt compounds and Optical brightener but still the color of the polyester had the yellow tinge. Incorporation of extra cobalt and other toners reduce other properties of polymer such as whiteness (L), filterability etc.
U.S. Pat. No. 6,143,837 deals with a process for preparation of polyester resins utilizing Ti alkoxides, acetylacetonates, dioxides and phosphites. Here, though the Ti catalyst is about four times more active than Sb catalyst, in order to overcome the problem of yellow coloration in the polyester addition of cobalt compounds other organic colorants have been suggested. Also the low reactivity of the Ti based catalyst towards solid-state polycondensation (SSP) has been overcome by aromatic dianhydride of a tetra carboxylic acid like Pyromellitic Dianhydride as an additive.
U.S. Pat. No. 6,372,879 deals with polyester polycondensation with Ti based catalyst along with a catalyst enhancer. Typically the composition consisted of titanyl oxalate catalyst and a metallic oxalate catalyst enhancer and as an option Sb based co-catalyst. Here the Titanyl oxalates consisted of a combination of lithium or potassium titanyl oxalates with Sb trioxide, triacetate or trisglycoxide.
U.S. Pat. No. 3,951,886 describe polyester polycondensation with titanyl oxalates leading to polyester with yellow coloration which has been compensated to some extent by adding cobalt based compounds.
U.S. Pat. No. 6,500,915 deals with polyester resin produced with a series of catalysts containing a Ti compound and restricting the addition level such that the rate and color of the polyester are not affected.
U.S. Pat. No. 6,541,598 use a composition consisting of a Ti compound and a complexing agent based on a phosphorous compound and polyester with better color has been claimed.
U.S. Pat. Nos. 6,723,768 & 5,453,479 though use Titanium compounds at low levels like 1 to 30 ppm the process is for a blend of polyester and polycarbonate. Also the catalyst used is a complex of a phosphorous compound and a titanium compound with a molar excess of titanium.
U.S. Pat. No. 6,787,630 deals with titanium catalysts for polyesterification. The process involves esterification followed by the addition of complexing agents comprising phosphorous based compounds followed by cobalt compounds and subsequently a titanium compound is added. Additionally a branching agent and optical brightener are also added if needed. In this work no process time data is mentioned and also the final color of the polymer as represented by the ‘L’ and ‘b’ values needs improvement. Their ‘L’ values are in the range 62 to 67 and most of their ‘b’ values are in the range 0.77 to 5.89 even in cases where optical brighteners have been used.
EP 1413593 (CA 2451994) uses a product of tetra alkyl titanium compounds along with a phosphorous based compound and an aromatic carboxylic acid and claims a polyester having high ‘L’ and low ‘b’ values with reduced acetaldehyde.